Laboratory study of initial sea-ice growth: properties of grease ice and nilas

We investigate initial sea-ice growth in an ice-tank study by freezing an NaCl solution of about 29 g kg−1 in three different setups: grease ice grew in experiments with waves and in experiments with a current and wind, while nilas formed in a quiescent experimental setup. In this paper we focus on the differences in bulk salinity, solid fraction and thickness between these two ice types. The bulk salinity of the grease-ice layer in our experiments remained almost constant until the ice began to consolidate. In contrast, the initial bulk-salinity evolution of the nilas is well described by a linear decrease of about 2.1 g kg−1 h−1 independent of air temperature. This rapid decrease can be qualitatively understood by considering a Rayleigh number that became maximum while the nilas was still less than 1 cm thick. Comparing three different methods to measure solid fraction in grease ice based on (a) salt conservation, (b) mass conservation and (c) energy conservation, we find that the method based on salt conservation does not give reliable results if the salinity of the interstitial water is approximated as being equal to the salinity of the underlying water. Instead the increase in salinity of the interstitial water during grease-ice formation must be taken into account. In our experiments, the solid fraction of grease ice was relatively constant with values of 0.25, whereas it increased to values as high as 0.50 as soon as the grease ice consolidated at its surface. In contrast, the solid fraction of the nilas increased continuously in the first hours of ice formation and reached an average value of 0.55 after 4.5 h. The spatially averaged ice thickness was twice as large in the first 24 h of ice formation in the setup with a current and wind compared to the other two setups, since the wind kept parts of the water surface ice free and therefore allowed for a higher heat loss from the water. The development of the ice thickness can be reproduced well with simple, one dimensional models that only require air temperature or ice surface temperature as input.publishedVersio

Recent wind driven high sea ice area export in the Fram Strait contributes to Arctic sea ice decline

Arctic sea ice area has been decreasing for the past two decades. Apart from melting, the southward drift through Fram Strait is the main ice loss mechanism. We present high resolution sea ice drift data across 79° N from 2004 to 2010. Ice drift has been derived from radar satellite data and corresponds well with variability in local geostrophic wind. The underlying East Greenland current contributes with a constant southward speed close to 5 cm s−1, and drives around a third of the ice export. We use geostrophic winds derived from reanalysis data to calculate the Fram Strait ice area export back to 1957, finding that the sea ice area export recently is about 25% larger than during the 1960's. The increase in ice export occurred mostly during winter and is directly connected to higher southward ice drift velocities, due to stronger geostrophic winds. The increase in ice drift is large enough to counteract a decrease in ice concentration of the exported sea ice. Using storm tracking we link changes in geostrophic winds to more intense Nordic Sea low pressure systems. Annual sea ice area export likely has a significant influence on the summer sea ice variability and we find low values in the 1960's, the late 1980's and 1990's, and particularly high values during 2005–2008. The study highlights the possible role of variability in ice export as an explanatory factor for understanding the dramatic loss of Arctic sea ice during the last decadespublishedVersio

Small-scale dynamics of the under-ice boundary layer

In ice covered polar regions, the interaction between ocean, ice and atmosphere is an important component in the complex climate system. Exchange of heat, mass and momentum occurs across the boundary layers, both in the ocean and in the atmosphere, hence understanding the involved processes are crucial in order to determine future climate. In this thesis dynamics and thermodynamics of the under-ice boundary layer are investigated based on measurements of turbulent fluxes in close proximity to the ice/ocean interface and microstructure profiling of the upper ocean. The topic is addressed in four papers which focus on exchange processes at the ice/ocean interface as well as regional measurements of turbulence and turbulent fluxes in ice covered areas around Spitsbergen and in the Weddell Sea. High rates of melting are often encountered as sea ice drifts into water with temperatures well above freezing, which may be typical of the marginal ice zones. It has been shown in previous studies that these melting rates are limited by double diffusive effects in a thin layer close the ice/ocean interface. In this study, turbulent fluxes from the under-ice boundary layer are used to show that double diffusive effects are important for the melting rates and show that the strength of this double diffusion is close to the range suggested by previous studies. It is also shown that by not considering double diffusive effects at the boundary, melting rates are overestimated by up to several cm per day. By analyzing the conditional statistics of the Reynolds stress in the boundary layer it is found that the main fraction of the stress comes from high turbulence events, so called “sweeps” and “ejections”, which is consistent with boundary layer flows in other environments. Closest to the ice, the sweeps are found to be more intense than further away from the interface, which can be related to the observed increase in friction velocity with depth. The West Spitsbergen Current transports Atlantic Water, which is the main source of salt and heat to the Arctic. This study presents measurements obtained during 6 drifting experiments northwest of Spitsbergen where the West Spitsbergen Current enters the Arctic and which also is an area of substantial air/sea/sea exchange. Heat fluxes within or in close proximity of the main branches of the West Spitsbergen Current are O(100) W m-², due to high mixed layer temperatures and large ice drift. Over the shelf areas high mixing and turbulent fluxes are observed due to tidal effects and interaction with topography. Heat fluxes averaged over the different water masses found in the area show that turbulent heat flux decreases with increasing distance from the surface. Hence it indicates that Atlantic Water is not the main source for vertical mixing of heat. A major contribution to mixed layer heat content is found to be horizontal advection and entrainment of water from below. In the far Southern Atlantic, the Weddell Sea is another important site of ocean and atmosphere interaction. In the area of Maud Rise, a topographic feature in the eastern Weddell Sea, the water column is only weakly stable making it susceptible to deep convection. This study presents wintertime mixed layer turbulence measurements obtained during two ice drift over the Maud Rise. Heat fluxes were comparable to earlier studies in the same area and could be estimated from the mean properties of the mixed layer. The under-ice roughness was estimated to be very smooth and comparison with a one dimensional steady state model suggests that this is a local effect and not representative for the entire ice floe. The main source of turbulent kinetic energy is velocity shear from the ice, however in some periods horizontal heterogeneity in water masses, ice topography and open water fraction can affect the stability and introduce upstream sources of turbulent kinetic energy

Turbulence and heat exchange under ice

Turbulent fluxes of heat and salt were measured under sea ice at four different locations around Spitsbergen. In Kongsfjorden on West Spitsbergen additional measurements of heat fluxes in the ice and in the atmosphere were done and compared in an air/sea/ice heat budget. Ocean heat flux in Kongsfjorden is about 13 W/m2 and comparison with the other heat fluxes at the ice/ocean interface shows a good agreement. From the heat budget at the ice/ocean interface, the ice growth during three subsequent days in March is calculated to be 4.4 cm. During the same three days the ice growth was measured to be 3.5 cm. The conductive heat flux in the ice is determined by the temperature gradient and the thermal conductivity of sea ice and the ice temperature is calculated from the measured convergence/divergence of conductive heat and absorption of short wave radiation. When the calculated ice temperature in Kongsfjorden is compared with the measured temperature, it shows that the best agreement occurs with a slight reduction of the thermal conductivity of sea ice (~10 – 15%). Turbulent fluxes of heat are also measured in Van Mijenfjorden and in outer parts of Storfjorden. At both locations there are only small amounts of heat in the water column and measured heat fluxes are of order 1 W/m2. Correspondingly, the turbulent fluxes of salt are small and of order -10-6 m/s psu , indicating small ice growth rates. In the so called Whaler’s Bay area north of Spitsbergen, the influence of the West Spitsbergen Current (WSC) is large. The WSC brings relatively warm water along the continental slope of Spitsbergen, resulting in large amounts of heat in the water column. In this area, heat fluxes of order 210 W/m2 were measured 1 m below the ice. Comparison with the conductive heat flux in the ice indicates melting rates of order 5 cm/day. Also the measured turbulent salinity flux in this area shows large rates of melting, about 3.5 cm/day. For all locations, a turbulent exchange coefficient for heat, the turbulent Stanton number, is calculated and the resulting Stanton numbers are in the range 0.006 – 0.007 for the locations Kongsfjorden, Van Mijenfjorden and Whaler’s Bay

Laboratory study of initial sea-ice growth: properties of grease ice and nilas

We investigate initial sea-ice growth in an ice-tank study by freezing an NaCl solution of about 29 g kg−1 in three different setups: grease ice grew in experiments with waves and in experiments with a current and wind, while nilas formed in a quiescent experimental setup. In this paper we focus on the differences in bulk salinity, solid fraction and thickness between these two ice types. The bulk salinity of the grease-ice layer in our experiments remained almost constant until the ice began to consolidate. In contrast, the initial bulk-salinity evolution of the nilas is well described by a linear decrease of about 2.1 g kg−1 h−1 independent of air temperature. This rapid decrease can be qualitatively understood by considering a Rayleigh number that became maximum while the nilas was still less than 1 cm thick. Comparing three different methods to measure solid fraction in grease ice based on (a) salt conservation, (b) mass conservation and (c) energy conservation, we find that the method based on salt conservation does not give reliable results if the salinity of the interstitial water is approximated as being equal to the salinity of the underlying water. Instead the increase in salinity of the interstitial water during grease-ice formation must be taken into account. In our experiments, the solid fraction of grease ice was relatively constant with values of 0.25, whereas it increased to values as high as 0.50 as soon as the grease ice consolidated at its surface. In contrast, the solid fraction of the nilas increased continuously in the first hours of ice formation and reached an average value of 0.55 after 4.5 h. The spatially averaged ice thickness was twice as large in the first 24 h of ice formation in the setup with a current and wind compared to the other two setups, since the wind kept parts of the water surface ice free and therefore allowed for a higher heat loss from the water. The development of the ice thickness can be reproduced well with simple, one dimensional models that only require air temperature or ice surface temperature as input

Recent wind driven high sea ice area export in the Fram Strait contributes to Arctic sea ice decline

Arctic sea ice area has been decreasing for the past two decades. Apart from melting, the southward drift through Fram Strait is the main ice loss mechanism. We present high resolution sea ice drift data across 79° N from 2004 to 2010. Ice drift has been derived from radar satellite data and corresponds well with variability in local geostrophic wind. The underlying East Greenland current contributes with a constant southward speed close to 5 cm s−1, and drives around a third of the ice export. We use geostrophic winds derived from reanalysis data to calculate the Fram Strait ice area export back to 1957, finding that the sea ice area export recently is about 25% larger than during the 1960's. The increase in ice export occurred mostly during winter and is directly connected to higher southward ice drift velocities, due to stronger geostrophic winds. The increase in ice drift is large enough to counteract a decrease in ice concentration of the exported sea ice. Using storm tracking we link changes in geostrophic winds to more intense Nordic Sea low pressure systems. Annual sea ice area export likely has a significant influence on the summer sea ice variability and we find low values in the 1960's, the late 1980's and 1990's, and particularly high values during 2005–2008. The study highlights the possible role of variability in ice export as an explanatory factor for understanding the dramatic loss of Arctic sea ice during the last decade

Wintertime mixed layer measurements at Maud Rise, Weddell Sea

The article of record as published may be found at http://dx.doi.org/10.1029/2008JC005141.Sea ice plays a crucial role in the exchange of heat between the ocean and the atmosphere, and areas of intense air-sea-ice interaction are important sites for water mass modification. The Weddell Sea is one of these sites where a relatively thin first-year ice cover is constantly being changed by mixing of heat from below and stress exerted from the rapidly changing and intense winds. This study presents mixed layer turbulence measurements obtained during two wintertime drift stations in August 2005 in the eastern Weddell Sea, close to the Maud Rise seamount. Turbulence in the boundary layer is found to be controlled by the drifting ice. Directly measured heat fluxes compare well with previous studies and are well estimated from the mixed layer temperatures and mixing. Heat fluxes are also found to roughly balance the conductive heat flux in the ice; hence, little freezing/melting was observed. The under-ice topography is estimated to be hydraulically very smooth; comparison with a steady 1-D model shows that these estimates are made too close to the ice-ocean interface to be representative for the entire ice floe. The main source and sink of turbulent kinetic energy are shear production and dissipation. Observations indicate that the dynamics of the under-ice boundary layer are influenced by a horizontal variability in mixed layer density and an increasing amount of open leads in the area.Naval Postgraduate SchoolNSFThe work of M. G. McPhee was supported by the National Science Foundation through grants OPP-0337159 and OPP- 0739371, and the work of J. H. Morison was supported by grant OPP- 0337751. The Naval Postgraduate School is acknowledged for the support for MaudNESS through NSF grant OPP-0338020. This is publication A247 from the Bjerknes Centre for Climate Research

A transitioning Arctic surface energy budget: the impacts of solar zenith angle, surface albedo and cloud radiative forcing

Snow surface and sea-ice energy budgets were measured near 87.5°N during the Arctic Summer Cloud Ocean Study (ASCOS), from August to early September 2008. Surface temperature indicated four distinct temperature regimes, characterized by varying cloud, thermodynamic and solar properties. An initial warm, melt-season regime was interrupted by a 3-day cold regime where temperatures dropped from near zero to -7°C. Subsequently mean energy budget residuals remained small and near zero for 1 week until once again temperatures dropped rapidly and the energy budget residuals became negative. Energy budget transitions were dominated by the net radiative fluxes, largely controlled by the cloudiness. Variable heat, moisture and cloud distributions were associated with changing air-masses. Surface cloud radiative forcing, the net radiative effect of clouds on the surface relative to clear skies, is estimated. Shortwave cloud forcing ranged between -50 W m-2 and zero and varied significantly with surface albedo, solar zenith angle and cloud liquid water. Longwave cloud forcing was larger and generally ranged between 65 and 85 W m-2, except when the cloud fraction was tenuous or contained little liquid water; thus the net effect of the clouds was to warm the surface. Both cold periods occurred under tenuous, or altogether absent, low-level clouds containing little liquid water, effectively reducing the cloud greenhouse effect. Freeze-up progression was enhanced by a combination of increasing solar zenith angles and surface albedo, while inhibited by a large, positive surface cloud forcing until a new air-mass with considerably less cloudiness advected over the experiment area. © 2010 Springer-Verlag

The Arctic Summer Cloud-Ocean Study (ASCOS): Overview and experimental design

The climate in the Arctic is changing faster than anywhere else on Earth. Poorly un-derstood feedback processes relating to Arctic clouds and aerosol-cloud interactionscontribute to a poor understanding of the present changes in the Arctic climate system,and also to a large spread in projections of future climate in the Arctic. The problem is exacerbated by the paucity of research-quality observations in the central Arctic. Im-proved formulations in climate models require such observations, which can only comefrom measurements in-situ in this difficult to reach region with logistically demandingenvironmental conditions.The Arctic Summer Cloud-Ocean Study (ASCOS) was the most extensive central Arctic Ocean expedition with an atmospheric focus during the International Polar Year(IPY) 2007–2008. ASCOS focused on the study of the formation and life cycle of low-level Arctic clouds. ASCOS departed from Longyearbyen on Svalbard on 2 August andreturned on 9 September 2008. In transit into and out of the pack ice, four short re-search stations were undertaken in the Fram Strait; two in open water and two in the marginal ice zone. After traversing the pack-ice northward an ice camp was set up on12 August at 87◦21′N 01◦29′W and remained in operation through 1 September, drift-ing with the ice. During this time extensive measurements were taken of atmosphericgas and particle chemistry and physics, mesoscale and boundary-layer meteorology,marine biology and chemistry, and upper ocean physics. ASCOS provides a unique interdisciplinary data set for development and testing ofnew hypotheses on cloud processes, their interactions with the sea ice and ocean andassociated physical, chemical, and biological processes and interactions. For exam-ple, the first ever quantitative observation of bubbles in Arctic leads, combined withthe unique discovery of marine organic material, polymer gels with an origin in the ocean, inside cloud droplets suggest the possibility of primary marine organically de-rived cloud condensation nuclei in Arctic stratocumulus clouds. Direct observations ofsurface fluxes of aerosols could, however, not explain observed variability in aerosol concentrations and the balance between local and remote aerosols sources remainsopen. Lack of CCN was at times a controlling factor in low-level cloud formation, andhence for the impact of clouds on the surface energy budget. ASCOS provided de-tailed measurements of the surface energy balance from late summer melt into theinitial autumn freeze-up, and documented the effects of clouds and storms on the surface energy balance during this transition. In addition to such process-level studies, theunique, independent ASCOS data set can and is being used for validation of satelliteretrievals, operational models, and reanalysis data sets.ISSN:1680-7375ISSN:1680-736